1. Field of the Invention
The present invention relates to an organic light emitting diode display and a driving method thereof, and more particularly to an organic light emitting diode display that is adaptive for increasing a display quality by improving ability of a pixel to express a gray scale, and a driving method thereof.
2. Description of the Related Art
Recently, there have been developed various flat panel display devices capable of decreasing their weight and bulk, which are regarded as disadvantages of a cathode ray tube. Such flat panel display devices include a liquid crystal display (hereinafter, referred to as “LCD”), a plasma display panel (hereinafter, referred to as “PDP”), and an electroluminescence device, etc.
The PDP has been regarded as a device having advantages of light weight and thin profile, and adaptive for making a large-dimension screen, as it has a simple structure and can be implemented by relatively simple manufacturing process. However, the PDP has disadvantages of a low luminous efficiency, a low brightness, and high power consumption. An active matrix LCD to which a thin film transistor (hereinafter, referred to as “TFT”) is applied as a switching device is difficult to be made large-sized because it is manufactured by using a semiconductor process. But, the demand for the LCD is continuously increasing since the LCD is mainly used as a display device of a notebook computer. In comparison with this, the electroluminescence device is broadly classified into an inorganic electroluminescence device and an organic light emitting diode device in accordance with a material of a luminous layer thereof. The electroluminescence device is a self-luminous device which emits light for itself, and has an advantage in that it has fast response speed, high luminous efficiency, high brightness and wide viewing angle.
The organic light emitting diode device includes an anode electrode formed of a transparent conductive layer on a glass substrate, and an organic compound layer and a cathode electrode that are sequentially disposed on the anode electrode, as shown in FIG. 1. Herein, the cathode electrode is formed of a conductive metal.
The organic compound layer includes a hole injection layer HIL, a hole transport layer HTL, an emission layer EML, an electron transport layer ETL, and an electron injection layer EIL.
If a drive voltage is applied to the anode electrode and the cathode electrode, holes in the hole injection layer HTL and electrons in the electron injection layer respectively move to the emission layer EML to excite the emission layer EML. And, as a result, the emission layer EML emits a visible light. In this way, a picture or an image can be displayed by using the visible light generated from the emission layer EML.
The organic light emitting diode device has been applied to a passive matrix type display device and an active matrix type display device that uses TFTs as switching devices. The passive matrix type selects a pixel in accordance with a current applied to an anode electrode and a cathode electrode which perpendicularly cross each other. On the other hand, the active matrix type selects a pixel by selectively turns-on the TFTs, and maintains the pixel to emit light by using a voltage kept in a storage capacitor.
In a LTPS (Low Temperature Poly Silicon) active matrix type display among them, which is manufactured by using an ELA (Excimer Laser Annealing), a characteristics of a TFT formed at an adjacent pixel region is changed according to a change of a line beam energy that is applied during a crystallization process. As a result, such a change of characteristics of a TFT device causes non-uniformity in brightness between adjacent pixels. In an active matrix type display employing an ELA LTPS substrate, a variety of compensation driving methods are applied in order to overcome the non-uniformity of brightness between adjacent pixels.
The compensation method is largely classified into an analog type compensation method and a digital type compensation method. The analog type compensation method uses a saturation region of a driving TFT, which is formed in a pixel, to overcome a change of a driving current in the pixel. On the other hand, the digital type compensation method uses a driving TFT simply as a switching device, and is able to overcome non-uniformity of brightness, as a change of characteristics of the driving TFT is slight compared to a saturation region thereof.
However, the digital type compensation method causes another problems relating to picture quality such as a flickering and a false counter, etc., and requires characteristics of an organic light emitting diode device that is adaptive for the digital type compensation method.
The analog type compensation method is largely classified into a voltage programmed driving method and a current programmed driving method. Herein, the voltage programmed driving method overcomes only a change in a threshold voltage among non-uniform parameters of a TFT. On the other hand, the current programmed driving method can overcome changes of a threshold voltage and mobility. The voltage programmed driving method directly controls a gate voltage of a driving TFT by using a data driving circuit of voltage driving type. On the other hand, the current programmed driving method lets a current corresponding to a gray scale to be displayed flow through a pixel during a data programming period by using a data driving circuit of current type. And, the current programmed driving method sets a gate voltage of the driving TFT that can controls an amount of driving current by using the current flowing through a pixel during a light emitting period thereby overcoming non-uniformity of brightness caused by differences between TFTs formed in adjacent pixels. Such a current programmed driving method can be classified into a sink type and a source type depending upon a configuration of a data driving circuit and a type of pixel that is matched with the technical configuration of the data driving circuit.
FIG. 2 is a block diagram of an organic light emitting diode display which is driven in a current sink type of the related art, and FIG. 3 is an equivalent circuit diagram showing any one of a plurality of pixels in FIG. 2.
FIG. 2 and FIG. 3, an organic light emitting diode display of the related art includes an organic light emitting diode display panel 16, a gate driving circuit 18, a current sink type data driving circuit 20, and a timing controller 24. Herein, the organic light emitting diode display panel 16 has pixels 22 which are arranged in each crossing part of the gate lines GL and the data lines DL. The gate driving circuit 18 drives the gate line GL. The current sink type data driving circuit 20 drives the data lines DL. The timing controller 24 controls the gate driving circuit 18 and the current sink type data driving circuit 20.
The timing controller 24 re-arranges video signals and supplies them to the current sink type data driving circuit 20. Also, the timing controller 24 generates a plurality of control signals to control driving timings of the current sink type data driving circuit 20 and the gate driving circuit 18.
The gate driving circuit 18 sequentially supplies a gate signal to the gate lines GL in response to a control signal from the timing controller 24.
The current sink type data driving circuit 20 receives current signals having current levels corresponding to video signals and sinks them to a low-level potential voltage source (not shown), thereby driving corresponding pixels 22 in response to control signals from the timing controller 24.
Each of the pixels 22 emit light in accordance with a driving signal to display a gray scale corresponding to a video signal. To this end, each of the pixels 22 includes an organic light emitting diode device OLED, a driving TFT DT, a programming TFT PT, first and second switch TFTs ST1 and ST2, and a storage capacitor Cst, as shown in FIG. 3. Each of the pixels 22 sinks a corresponding current signal through a constant current source Idata for a programming period to charge a control voltage that controls an amount of luminescence of the organic light emitting diode device OLED. Then, each of the pixels 22 makes the organic light emitting diode device OLED emit light by using a driving current according to the control voltage to display a gray scale corresponding to a video signal.
FIG. 4A is an equivalent circuit diagram of a pixel for the programming period, and FIG. 4B is an equivalent circuit diagram of a pixel for a light emitting period.
Referring to FIG. 4A, the first and second switch TFTs ST1 and ST2 are turned-on in response to a scanning pulse having a high logical voltage to allow a current, which is sunk by the constant current source Idata, to be passed from a high-level potential voltage source VDD, through the programming TFT PT and the second switch TFT ST2, to a low-level power voltage source VSS for the programming period. By the such current flow, a voltage Vg charged into a node n1 is stored in the storage capacitor Cst and is maintained for the light emitting period. Referring to FIG. 4B, the first and second switch TFTs ST1 and ST2 are turned-off in response to a scanning pulse having a low logical voltage to stop a current sink operation by the constant current source Idata. In this case, the driving TFT DT is controlled by a voltage difference Vgs between a first node voltage Vg stored at the storage capacitor Cst and a high-level driving voltage VDD thereby adjusting an amount of driving current which flows into the organic light emitting diode OLED, through the high-level potential voltage source VDD, the programming TFT PT, and the second switch TFT ST2.
However, for the organic light emitting diode display of the related art, shown in FIG. 3 and FIG. 4, to accurately realize a gray scale, it should be preconditioned that all characteristics of the programming TFT PT (a threshold voltage, mobility, a constant determined by mobility and a parasitic capacitance, etc) are the same as those of the driving TFT DDT. This is because the first node voltage Vg, which is set for the programming period, reflects only characteristics of the programming TFT PT as shown in FIG. 4A. If the first node voltage Vg charged during the programming period is different with a gate voltage of the driving TFT DT during the light emitting period that follows the programming period, a desired gray scale can not be displayed. Herein, the gate voltage of the driving TFT DT during the light emitting period determines an amount of driving current. Furthermore, in order to increase an ability of charging a current during the programming period, the programming TFT PT is designed to have a size several times larger than the driving TFT DT. Because of this, characteristics discrepancy between the programming TFT PT and the driving TFT DT is deepened. This can be represented by Mathematical Formula 1, as below.
                    Ioled        =                                            Kd                              Kd                +                Ks                                      ⁡                          [                                                                                          1                      +                                              (                                                                                                            μ                              ⁢                                                                                                                          ⁢                              d                                                        -                                                          μ                              ⁢                                                                                                                          ⁢                              s                                                                                                            μ                            ⁢                                                                                                                  ⁢                            s                                                                          )                                            +                                                                                                                                                          (                                                                              Vthd                            -                            Vths                                                    Vths                                                )                                            2                                                                                  ]                                ⁢          Idata                                    [                  Mathematical          ⁢                                          ⁢          Formula          ⁢                                          ⁢          1                ]            Herein, Ioled represents a driving current, Idata represents a current which is sunk through the constant current source, Kd represents a constant which is determined by mobility and a parasitic capacitance (Hereinafter, referred to as “a natural constant”) of a driving TFT DT, Ks represents a natural constant of a programming TFT PT, μd represents mobility of the driving TFT DT, μs represents mobility of the programming TFT PT, Vthd represents a threshold voltage of the driving TFT DT, and Vths represents a threshold voltage of the programming TFT PT, (Kd+Ks)/Kd represents a scaling ratio (Idata/Ioled) that is for increasing an ability of charging a current during the programming period, and
      (                            μ          ⁢                                          ⁢          d                -                  μ          ⁢                                          ⁢          s                            μ        ⁢                                  ⁢        s              )    +            (                        Vthd          -          Vths                Vths            )        2  is a mismatching factor that represents characteristics discrepancy between the driving TFT DT and the programming TFT PT.
In Mathematical Formula 1, if a channel width of the programming TFT PT is 20 μm, a channel length of the programming TFT PT is 10 μm, a threshold voltage of the programming TFT PT is −2.2V and mobility of the programming TFT PT is 50 cm2/Vs, and a channel width of the driving TFT DT is 5 μm, a channel length of the driving TFT DT is 10 μm, a threshold voltage of the driving TFT DT is −2.0V and mobility of the driving TFT DT is 55 cm2/Vs, then a scaling ratio is 25/5 (i.e. five times), and a mismatching ratio between the driving TFT DT and the programming TFT PT is about 10.8%.
However, such a high mismatching ratio exceeding 10% reduces a compensating ability of a current during the programming period and, as a result, decreases ability to express a gray scale during the following light emitting period, thereby reducing a display quality.